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1. Effective Time Scale of the Northern Hemisphere Winter Circulation Waviness

   https://doi.org/10.1175/JCLI-D-24-0236.1  

   Chen, Gang; Nie, Yu (February 2025, Journal of Climate)

   Abstract Midlatitude weather extremes such as blocking events and Rossby wave breaking are often related to large meridional shifts in the westerly jet stream. Numerous diagnostic methods have been developed to characterize these weather events, each emphasizing different yet interrelated aspects of circulation waviness, including identifying large-amplitude ridges or persistent anomalies in geopotential height. In this study, we introduce a new metric to quantify the circulation waviness in terms of effective time scale. This is based on the Rossby wave packet from the one-point correlation map of anomalous meridional wind, applicable to jet waviness involving multiple wavenumbers. Specifically, we estimate the intrinsic frequency of Rossby waves and decay time scale of wave amplitude in the reference frame moving at the local time mean zonal wind. The resulting effective time scale, derived from linear theory, serves as a proxy for the eddy mixing time scale in jet meandering. Remarkably, its spatial distribution roughly resembles that of circulation waviness in the Northern Hemisphere winter as depicted by local wave activity (LWA). In the high-latitude regions characterized by weak zonal winds, the long time scale in waviness aligns with large values in LWA. By contrast, short waviness time scales in subtropical jet regions correspond to the suppressed amplitude in waviness despite large values in eddy kinetic energy (EKE). Furthermore, the effective time scale in waviness largely captures the interannual variability of LWA in observations and its projected future changes in climate model simulations. Thus, this relation between the waviness time scale and zonal wind provides a physical mechanism for understanding how zonal wind changes impact regional weather patterns in a changing climate. Significance StatementThe purpose of this study is to better understand what controls weather extremes in midlatitude regions such as blocking events and Rossby wave breaking. We introduce a novel concept, the effective time scale of jet stream meandering, which sheds light on these phenomena. Through analyzing Rossby waves in the reference frame moving at the local time mean zonal wind, we derive a scaling relation between circulation waviness and eddy mixing time scale. Our findings reveal that this time scale closely mirrors the spatial distribution of circulation waviness in the Northern Hemisphere winter. Importantly, it captures interannual variability and climate change responses. These insights provide a physical basis for understanding how changes in zonal wind impact regional weather patterns in observations and climate models. 

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2. Local wave-activity analysis of atmospheric blocks in the Northern Hemisphere winter

   https://doi.org/10.1175/JCLI-D-24-0232.1  

   Barpanda, Pragallva; Nakamura, Noboru (April 2025, Journal of Climate)

   Abstract Atmospheric blocking entails a persistent, anomalous meandering of the jet stream that disrupts the eastward migration of transient eddies in the midlatitudes. Here we analyze a large number of blocking (and blocking-like) events in the Northern Hemisphere winter with the ERA5 reanalysis through the lens of vertically-averaged wave-activity budget. By applying a feature tracking algorithm, large-valued wave-activity anomalies that persist for 4 days or longer at a given location are identified as blocks, and block-centered composites are constructed for the wave-activity budget through the lifecycle of blocks. The identified events share commonly recognized features of blocking. The majority of the persistent events occur in clusters collocated with the quasi-stationary ridge associated with the Atlantic and the Pacific storm track. Frequency of persistent blocks is higher (lower) in regions where the ‘carrying capacity’ of the jet stream is lower (higher). A very low carrying capacity for the transient waves leads to a large population of blocks over Europe. The composite lifecycle of persistent blocks shows that convergence (divergence) of the zonal flux of wave-activity dominates the budget during the onset (decay) phase of the block, while the eddy-induced wind plays a crucial role of suppressing the zonal flux during the maturation period. Our finding broadly supports the ‘traffic jam’ hypothesis of Nakamura and Huang as a common mechanism of block formation, although there is vast diversity in the actual manifestation of individual blocks. It is argued that carrying capacity is suited for estimating blocking probability rather than for making deterministic forecasts of blocking events. 

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3. Traffic Bottlenecks: Predicting Atmospheric Blocking With a Diminishing Flow Capacity

   https://doi.org/10.1029/2024GL111035  

   Yan, Xingjian; Wang, Lei; Gerber, Edwin P; Castañeda, Valentina; Ho, Ka Ying (October 2024, Geophysical Research Letters)

   Abstract Atmospheric blocking is characterized by persistent anticyclones that “block” the midlatitude jet stream, causing temperature and precipitation extremes. The traffic jam theory posits that blocking events occur when the Local Wave Activity flux, a measure of storm activity, exceeds the carrying capacity of the jet stream, leading to a pile up. The theory's efficacy for prediction is tested with atmospheric reanalysis by defining “exceedance events”, the time and location where wave activity first exceeds flow capacity. The theory captures the Northern Hemisphere winter blocking climatology, with strong spatial correlation between exceedance and blocking events. Both events are favored not only by low carrying capacity (narrow roads), but also a downstream reduction in capacity (lane closures causing a bottleneck). The theory fails, however, to accurately predict blocking events in time. Exceedance events are not a useful predictor of an imminent block, suggesting that confounding factors explain their shared climatological structure. 

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4. Impact of Multidecadal Variability in Atlantic SST on Winter Atmospheric Blocking

   https://doi.org/10.1175/JCLI-D-19-0324.1  

   Kwon, Young-Oh; Seo, Hyodae; Ummenhofer, Caroline C.; Joyce, Terrence M. (February 2020, Journal of Climate)

   Recent studies have suggested that coherent multidecadal variability exists between North Atlantic atmospheric blocking frequency and the Atlantic multidecadal variability (AMV). However, the role of AMV in modulating blocking variability on multidecadal times scales is not fully understood. This study examines this issue primarily using the NOAA Twentieth Century Reanalysis for 1901–2010. The second mode of the empirical orthogonal function for winter (December–March) atmospheric blocking variability in the North Atlantic exhibits oppositely signed anomalies of blocking frequency over Greenland and the Azores. Furthermore, its principal component time series shows a dominant multidecadal variability lagging AMV by several years. Composite analyses show that this lag is due to the slow evolution of the AMV sea surface temperature (SST) anomalies, which is likely driven by the ocean circulation. Following the warm phase of AMV, the warm SST anomalies emerge in the western subpolar gyre over 3–7 years. The ocean–atmosphere interaction over these 3–7-yr periods is characterized by the damping of the warm SST anomalies by the surface heat flux anomalies, which in turn reduce the overall meridional gradient of the air temperature and thus weaken the meridional transient eddy heat flux in the lower troposphere. The anomalous transient eddy forcing then shifts the eddy-driven jet equatorward, resulting in enhanced Rossby wave breaking and blocking on the northern flank of the jet over Greenland. The opposite is true with the AMV cold phases but with much shorter lags, as the evolution of SST anomalies differs in the warm and cold phases. 

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5. Dynamics of the Southeast Indian Ridge storm track evaluated using a wave activity flux framework

   Exley, Alexis J; Donohue, Kathleen; Xu, Xiaobiao (April 2026, Journal of physical oceanography)

   Regions of elevated eddy kinetic energy (EKE) downstream from topographic ridge systems of the Southern Ocean are analogous to atmospheric storm tracks. Typically investigated using a 3-dimensional wave activity flux framework, dynamics of storm tracks are primarily governed by energy transfers between the mean and eddy field. Here, three years of high-resolution numerical model output is used to diagnose these eddy-mean field interactions and the physical mechanisms responsible for initiating and maintaining the Southeast Indian Ridge storm track in the Southern Ocean. Background dynamics are found to be primarily governed by the vertical wave activity flux, corresponding to the baroclinic conversion of energy. This term suggests ubiquitous baroclinic growth across the domain but largely enhanced in the crest of the standing meander. Temporal variability in array averaged EKE however is found to be dominated by the horizontal wave activity flux terms, corresponding to the barotropic conversion of energy. This variability is event driven and, when decomposed into synoptic stormtrack case studies, shown to be analogous to atmospheric downstream development, a process by which wave-train like propagation is sustained by barotropic growth, flux and decay of energy in the along stream direction. Anomalously large local increases in EKE are attributed to an extremely extended meander which obstructs the typical propagation of these barotropic perturbations. These major events are compared to an atmospheric omega block and hypothesized to significantly increase eddy heat and tracer fluxes and therefore play a sizable role in the eddy driven arm of the Southern Ocean overturning circulation. 

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